This invention relates to metal oxide powders useful in the production of various ceramic materials, and to processes for making such metal oxide ceramic powders, particularly metal oxide ceramic powders of fine particle size.
Fine particle size (0.1-1.0 micron diameter), metal oxide powders are highly desirable in ceramic product manufacturing. Sintering of these fine powders occurs at temperatures much lower than most larger-sized commercial powders.
The traditional methods for producing ceramic powders involve mechanically mixing the component oxides in their appropriate stoichiometric amounts, calcining the solution at high temperature to obtain a single phase, and mechanically milling the product to obtain the desired particle size. However, high temperature calcination promotes crystal growth, which is an undesirable effect, the desired end product being a fine-grained ceramic powder. On the other hand, mechanical milling often introduces unwanted impurities into the final ceramic powder product.
A number of somewhat complicated chemical methods have been developed to produce fine ceramic powders. Included are controlled hydrolysis and condensation of metal alkoxides (sol gel), flame oxidation, and spray drying of a colloidal suspension, freeze-drying, and coprecipitation.
In U.S. Pat. No. 3,330,697, a method of producing electrical capacitors is described employing lead and alkaline earth titanates, zirconates and niobates. These compounds are used in selected combinations and proportions to form resin intermediates. Calcination of the resin intermediates removes the organic material and leaves a metal oxide powder, ideally in a finely divided state. A list of compounds which can be used in this process are provided in column 2, lines 62-67.
In a subsequent article in Mat. Res. Soc. Symp. Proc., Vol. 32, pages 239-244 (1984), entitled, Preparation Of Strontium Titanate Ceramics In Internal Boundary Layer Capacitors By The Pechini Method, by Budd, et al., strontium titanate is described as one of the titanate perovskite materials that are widely used in the electronic ceramics industry. In the study described in the article, a liquid resin-forming technique, as described in the above U.S. Pat. No. 3,330,697 was used to prepare the strontium titanate powders.
In an article entitled, Preparation of Fine Organic Powders by Evaporative Decomposition Solutions, by D. M. Roy, et al., in Am. Ceram. Soc. Bull., Vol 56, at page 1023 (1977), the authors describe ceramic powder production by evaporative decomposition of solutions. Specifically, aluminum oxide and calcium aluminum oxide powders were prepared in laboratory scale furnaces using the EDS technique. Details of the furnace and atomizer assembly, respectively, employed in the EDS method are set forth in FIGS. 1 and 2 of the Roy article.
Accordingly, a need exists for a process for producing a ceramic powder, particularly a fine, single-phase ceramic powder, without the use of complex equipment such as the above-described EDS furnace, which can be produced in a direct manner with a minimum amount of skill and labor, and which provides a high-quality product.